Geothermal generation of electricity can be cited as a typical example of the utilization of subterranean heat. Existing methods of generation of electricity of this type include those in which natural steam heated by subterranean heat sources and emitted from underground is used, those in which hot water is emitted from underground or pumped from underground is used, and those in which steam of other working fluids is produced by natural steam or hot water emitted or pumped up from the earth as previously indicated. However, in all these methods, the natural steam and hot water is kept entirely at the earth's surface, for which reason, as it is used up, the steam must be discharged into the ground or the water must be discharged into rivers or ponds. However, natural steam and hot water of this kind contain harmful constituents and for this reason, pollution of the environment cannot be avoided when steam or water of this kind is discharged. Pipelines can be used to carry natural steam and hot water that has been brought to the earth's surface to the required location, or they can be introduced into a heat exchange apparatus. However, when pipelines or heat exchange apparatus of this kind are damaged, the natural steam and hot water may be dispersed in all directions, as the result of which pollution of the environment may occur.